Inclination Shallowing in Sedimentary Rocks: Evidence, Mechanism and Cause - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 41

sediment and the establishment of a clay fabric during
compaction is the reason for this behavior.
The observation of this behavior is signifi cant for
understanding inclination shallowing because of its
prevalence in all the laboratory compaction experi-
ments. The simplest explanation for the disparity in the
amount of inclination shallowing for the different
grain-size slurries is that the amount of inclination
shallowing is proportional to the amount of volume
lost during compaction. The clay-sized fraction experi-
enced up to 75% volume loss, the coarse-grained sedi-
ments only experienced about 12% volume loss and
the mixture experienced a 35% volume loss. Interest-
ingly, simply dripping slurry into the sample holder
used for laboratory compaction initially magnetized
the samples. While the slurries experienced a small
amount (6 - 8 ° ) of initial syn - depositional inclination
shallowing, this disappeared after the samples sat in
the laboratory fi eld for only about an hour; this sug-
gests post-depositional realignment of the hematite
grains. A separate re-deposition experiment with the
coarse-grained slurry did show signifi cant syn-
depositional inclination shallowing similar to that seen
by Tauxe & Kent (1984) if the sediment accumulation
rates were fast ( c. 5 mm/min). The fastest deposition
rates experienced the most shallowing (28°), but
slower rates saw little or no syn-depositional shallow-
ing (0.5 mm/min).
The laboratory compaction work set the stage for
work that was designed to search for inclination shal-
lowing in natural sedimentary rocks. Paleomagnetists'
experience with recent lake and marine sediments in
the 1960s, 1970s and 1980s suggested that inclina-
tion fl attening in sedimentary rocks was probably not
common or important. The inclination error seen in
the early re-deposition experiments that used mainly
varved glacial lake sediments was attributed to being
an artifact of the experiment's design. Either the fast
sediment accumulation rate or the drying used to
allow measurement of the re-deposited sediment was
the suspected cause of the inclination error. Paleomag-
netists also argued, as we saw in Chapter 3, that a
post-depositional remanence would remove the effects
of any syn-depositional inclination error even if a syn-
depositional error sometimes occurred in nature. The
laboratory compaction experiments showed that the
50% or larger volume loss at depths of hundreds of
meters in the sediment column could shallow the incli-
nation of both marine sediments and quite possibly a
DRM or an early-acquired chemical remanent mag-
netization (CRM) in hematite-bearing red beds. At this

point, workers started to look for inclination shallow-
ing in sedimentary rocks.

OBSERVATIONS OF INCLINATION
SHALLOWING IN SEDIMENTARY
ROCKS

Magnetite-bearing sediments and rocks

Most of the observations of inclination shallowing in
sediments and in sedimentary rocks were published in
the 1990s and 2000s. The studies discussed here and
listed in Table 4.1 are separate from the studies dis-
cussed in the next chapter that consider identifi cation
and correction of inclination shallowing by either the
anisotropy technique or the elongation-inclination
(EI) directional distribution technique (Tauxe & Kent
2004). Combining the two groups of studies gives a
large dataset showing that inclination shallowing has
occurred in both magnetite-bearing and hematite-
bearing sediments and sedimentary rocks.
The studies in Table 4.1 that observed inclination
shallowing in magnetite-bearing rocks can be divided
into two basic groups: freshly cored marine and
lake sediments and ancient sedimentary rocks. In the
late 1980s, two important studies fi rst unequivocally
showed inclination shallowing in marine sediments
downcore as porosity decreased due to burial com-
paction. Celaya & Clement (1988) suggested that
carbonate content was the culprit and observed that
the marine sediments with greater than 80% carbon-
ate content had signifi cant inclination shallowing
( f = 0.48). Sager & Singleton (1989) saw signifi cant
shallowing ( f = 0.62) in four out of ten cores drilled
around a salt dome in the northern Gulf of Mexico.
Arason & Levi (1990) documented inclination shal-
lowing downcore in DSDP Hole 578 in the Pacifi c
Ocean. The inclination shallows from 53° at the top of
the core, an accurate direction for the core's latitude,
by 6-8° at a depth of 120 m. Only a quarter of the
shallowing could be attributed to Pacifi c Plate motion
over the past 6.5 million years. The inclination shal-
lowing ( f = 0.75) can be correlated to a 3-4% porosity
decrease downcore, strongly supporting the conten-
tion that compaction-caused inclination shallowing
occurs in nature. McNeill (1997) saw evidence of incli-
nation shallowing from a core into the Great Bahama
Bank sediments. Some cores had very little shallowing
( f = 1.39 and 0.92) while others had more signifi cant
shallowing (f = 0.78 and 0.84).